Anti-theft and anti-pilferage surveillance systems have been extensively used to reduce the loss of unpaid goods in the retail merchandising industry. Such detection systems generally comprise two major components. First, a specially constructed label or tag is affixed to the goods and is activated to respond to electromagnetic energy for providing an indication of the label or tag, and thus the article of merchandise itself. Secondly, the detection system includes a source of electromagnetic energy, generally in the Very High Frequency (VHF) range, which is transmitted in the zone of detection. The detection system also includes a receiver for detecting changes in the transmitted energy due to a label passing through the detection zone. In this manner, articles which have not been paid for, and thus which still include a label, can be detected before a shoplifter exits the premises. Of course, when a sale of the article has been properly made, the label is either removed or deactivated, thereby preventing an alarm by the detection system.
In the anti-theft surveillance systems documented in the art, the mechanism generally employed for detecting the presence of the label or tag is the occurrence of a high frequency generated by label, which frequency is harmonically related to the frequencies transmitted within the zone of detection. For example, in U.S. Pat. No. 4,298,862, by Gregor et al., the label is constructed of a strip of amorphous ferromagnetic metal which produces magnetic fields at frequencies which are harmonics of the fundamental 8 KHz frequency generated by a transmitter within the zone of detection.
U.S. Pat. No. 4,527,152 by Scharr et al. discloses an anti-shoplifting system comprising a pair of electromagnetic coils located on each side of the zone of surveillance, and a special label with magnetic properties which is detectable in such zone. The coils are alternately driven in and out of phase with a fundamental frequency of 12.5 KHz to produce a magnetic field characterized by three vectors. The magnetic label includes a permeability of 100,000, and a coercive force of 0.05 Oersteds. The magnetic label produces a signal of approximately the 160th harmonic of the fundamental frequency. As the label passes through the zone of surveillance, a detector detects the harmonic signal and provides an alarm to signal that unauthorized merchandise associated with the label is passing through the zone.
The labels heretofore utilized with theft detection systems have required a high level of energy to satisfactorily operate in conjunction with the magnetic properties of the label. Because of the construction of the labels, a high level of magnetic energy was required to sufficiently saturate the ferromagnetic label material so that harmonic frequencies could be produced. As noted in the prior art patents, the energy transmitted by the detection systems occurred in the VHF band. The construction of labels or tags responsive to these frequencies and energy levels necessitated a substantial amount of ferromagnetic material in the tag. As a result, the labels tend to be large and difficult to attach to merchandise, as well as being lossy and inefficient in operation.
Further, previously developed detection systems have been limited to the VHF range, although higher level frequencies would provide advantages in detection. Such prior systems were so limited because higher frequencies would require higher energy levels, which would create practical manufacturing and operating problems, as well as possible health problems.
From the foregoing, it may be seen that a need has arisen for an improved anti-theft surveillance system which operates in the superhigh frequency range, and a label or tag which is smaller in size, less cumbersome and more easily manufactured.